There are many applications where a valve is provided in an air flow path to control the flow of the air, for example, in the ducting of an indoor critical environment (hospitals, laboratories, etc.) or in the ducting of an indoor non-critical environment (classrooms, conference rooms, etc.). Some conventional valves include an air flow station in the form of a cross flow sensor that includes two cross bars to measure total pressure and static pressure in order to determine the velocity pressure inside the valve. The velocity pressure is used to calculate the current air flow rate, and a damper inside the valve is rotated to provide the desired air flow rate.
Other conventional valves include a mechanical air regulator in the form of a cone-shaped element positioned in and movable in the valve's orifice. The cone-shaped element varies the size of an annular-shaped fluid flow path formed in the orifice. Due to the shape of the cone and the orifice, the pressure drop across the valve's orifice can be measured by the force exerted on the cone by the difference between the static pressure directly in front of and behind the cone caused by the increased air velocity behind the cone. The valve uses this force to act upon a variable rate spring located inside the cone, which connects the cone to the valve's shaft. The purpose of the spring is to provide a pressure-compensating action so that for a given position of the valve's shaft, the flow rate of the valve is constant or independent of pressure changes over some range of pressure drops across the valve. However, the actual air flow rate is derived from the position of the valve's shaft, not from sensor measurements of the static pressure in front of and behind the cone.
In both of these conventional configurations there is an air flow station or mechanical air regulator positioned inside the valve's orifice that interferes with a significant portion of the cross-sectional area available for air flow. When the air flow station or mechanical air regulator interferes with air flow, the air flow station and even the valve itself can become clogged with debris. In addition, both the cross flow sensor and the cone-spring configurations must be calibrated properly in order to accurately determine the actual air flow rate. If these devices are no longer calibrated properly, the valve must be accessed within the walls of the building to re-calibrate it.